This invention relates to a differential current distributing circuit.
In a known hall motor, for example, at least one hall element is arranged around a rotor having an alternate array of N and S poles so as to detect the momentary position of the rotor. A drive current for driving the rotor is supplied to one of at least two driving coils disposed around the rotor by using the detection signal from the hall element, causing the rotor to be rotated in a predetermined direction. FIG. 1 shows a conventional differential current distributing circuit which can be used for this purpose. An emitter-collector path of a transistor TR1 of a predetermined conductivity type, for example, a PNP type is connected between an output terminal of a constant current source IE and a first current output terminal 1. A transistor TR2 of the same conductivity type as the transistor TR1 is connected between the output terminal of the constant current source IE and a second current output terminal 2. A signal IN1 on one output terminal of the hall element is coupled to the base electrode of the transistor TR1 and a signal IN2 on the other output terminal of the hall element is coupled to the base electrode of the transistor TR2. FIG. 2 shows the characteristic curve of the differential current distributing circuit as shown in FIG. 1. The abscissa in FIG. 2 denotes the polarity and level of the input signal IN1 with the input signal IN2 as a reference. A "0" point on the abscissa is a point where the level of the input signal IN1 is equal to that of the input signal IN2. The arrow 3 in FIG. 2 shows the direction in which the input signal IN1 is positive with respect to the input signal IN2 and the arrow 4 in FIG. 2 shows the direction in which the input signal IN1 is negative with respect to the input signal IN2. The ordinate in FIG. 2 shows the amplitude of the distributed current. In the graph as shown in FIG. 2, I.sub.1 denotes a current value as obtained from the current output terminal 1 and I.sub.2 a current value as obtained from the current output terminal 2. In FIG. 2, I.sub.11 shows the value of the current I.sub.1 when the input signal IN1 has a negative value corresponding to a line 0-5 with respect to the input signal IN2, and I.sub.21 shows the value of the current I.sub.2 under the same conditions. When the current distributing circuit as shown in FIG. 1, is used as the drive circuit for the hall motor, the current I.sub.1 is supplied to one drive coil for the rotor and the current I.sub.2 is supplied to the other drive coil for the motor. The polarities of the output signals of the hall element, i.e., the input signals IN1 and IN2 to the current distributing circuit vary according to the rotation of the rotor. It is desirable that the output current of the constant current source IE be completely and accurately distributed into one drive coil or the other drive coil in response to the point of time at which the polarity of the input signal IN1 or IN2 varies from positive to negative or from negative to positive. As evident from FIG. 2, the current I.sub.1 and current I.sub.2 vary along a hyperbolic tangential curve corresponding to the level of the input signal IN1 with the input signal IN2 as a reference. Therefore, it is difficult to completely distribute the current I from the constant current source IE into either of the current output terminals 1 and 2 in sharp response to the time at which the polarity of the input signal varies.
There is the case where it is desired to detect the rotational direction of the rotor by a means (not shown) and to control the driving of the hall motor by the detection signal from a hall element. In this case, a plurality of hall elements are arranged around the rotor. The respective hall element produces an output signal corresponding to the rotational position of the rotor. The hall elements can be arranged such that the cycles of the position signals from the respective hall elements have the same cycle, but that the phases of the position signals are different from each other.
The reason why the rotational direction of the rotor can be detected by arranging, for example, two hall elements around the rotor will be explained below by referring to FIGS. 3A and 3B. Now suppose that in FIG. 3A a denotes the output waveform of the first hall element, that b denotes the output waveform of the second hall element and that when the rotor is rotated in the clockwise direction the output waveform b is lagged by an angle .theta. with respect to the output waveform a. At the rise of the waveform a the polarity of the waveform a is opposite to that of the waveform b (FIG. 3A). When the rotor is rotated in the counterclockwise direction, the waveform b is leaded by the angle .theta. with respect to the waveform a and at the rise of the output waveform a the waveforms a and b have the same polarity. In other words, at the rise of the output waveform a of the first hall element the polarity of the output waveform a is different from that of the output waveform b of the second hall element if the rotational direction of the rotor is different. It is to be noted that a first differential current distributing circuit is connected to the output ends of the first hall element and that a second differential current distributing circuit is connected to the output ends of the second hall element. The rotation direction of the rotor is detected by the outputs of the two hall elements and, when the distribution of the current is to be effected in the first current distributing circuit, the value of the output current I of the constant current source IE in the first current distributing circuit is varied by the output of the second hall element according to the result of the detection.
In such a case, it is desired that the current distributing circuit have the following characteristic. That is, it is desired that, according to the polarity of that input signal IN1 with respect to the input signal IN2 which corresponds to the instant at which the current of the current source IE begins to flow, the output current of the current source IE be completely distributed into either of the first current path including the first current output terminal 1 and second current path including the second current output terminal 2, and that, even if the polarity of the input signal IN1 varies with respect to the input signal IN2, the distributed current path remain established during the time period in which the current of the current source IE maintains a predetermined level.
Since, however, the current values I.sub.1 and I.sub.2 obtained from the output current terminals 1 and 2 vary along the hyperbolic tangential curve, as shown in FIG. 2, according to the polarity and magnitude of the input signal IN1 with respect to the input signal IN2, it is impossible to obtain the above-mentioned desired characteristic from the current distributing circuit as shown in FIG. 1.